Adherence of Entamoeba histolytica trophozoites to human colonic mucin glycoproteins, and contact-dependent killing of immune effector cells such as macrophages and neutrophils, require the activity of the galactose-specific adherence lectin. This amebic lectin also mediates resistance to serum lysis by interfering with the assembly of the membrane attack complex. The lectin is antigenically conserved in pathogenic zymodemes of E. histolytica, and is almost universally recognized by the humoral and cell mediated immune responses of individuals recovered from amebiasis. Immunization with the native lectin provides complete protection from the development of liver abscesses in 67% of immunized gerbils. Immunization of animals with the light subunit (expressed as an E. coli fusion protein) provides partial protection from amebic liver abscess. The lectin heterodimer has an apparently unprecedented mechanism of membrane anchorage: heavy subunit cDNA sequences predict it is transmembrane, while an acyl- glycosylphosphatidylinositol (GPI) anchor has been identified on the light subunit. Further study of the structure and function of the lectin is central to a molecular understanding of amebic pathogenesis. The goals of this proposal are to investigate the light subunit's role in adherence, cytolysis and serum resistance, and use monoclonal antibodies against pathogenic-specific epitopes on the lectin to develop new diagnostic tests for amebiasis. Two specific aims are proposed: 1. to produce and use light subunit- specific antibodies to probe its contributions to the biologic activities ascribed to the lectin; and 2. to test the practical utility of anti- lectin antibodies to rapidly and specifically diagnose infection by pathogenic E. histolytica. It is proposed that an understanding of the molecular structure and function of the E. histolytica adherence lectin will not only provide fundamental insights toward the pathogenesis of amebiasis, but will yield a rapid diagnostic test for the disease and a basis for rational vaccine development.